The mechanical properties of fiber-reinforced materials are dominated by the properties and distribution of the fiber constituent. Fiber distribution flaws of importance to composite integrity include such gross imperfections as missing or misaligned tape segments. The results of such errors can range from an improper thickness to warping of the component. Less important distribution flaws include such defects as improper tape-to-tape spacing, wrinkling of a tape, waviness, and the like.
To assure the structural reliability of composite parts, there is a need for an effective method of nondestructive examination. At the present time, even with automatic tape-laying machines, the inspection for the above-mentioned defects is performed visually prior to lay-up of the plies. Visual examination is a difficult and expensive process since the inspector must examine black composite tapes on a background of black graphite fibers. Several techniques have been suggested to detect fiber distribution flaws even after consolidation of the component. Included among such techniques is the addition to the preform of lead glass fibers which can be seen radiographically. Another suggested technique involves the use of eddy current measurements for graphite composites. Both of these methods have several disadvantages. Lead glass fibers provide no indication as to whether a composite is damaged in service, i.e., has ruptured plies; they only tell if plies are properly oriented. Also, lead glass fibers require special handling because of their low strength, are difficult to obtain, and are very expensive. The eddy current technique can be used only on graphite-reinforced composites and decreases in sensitivity with increasing part thickness.
It is an object of this invention, therefore, to provide a composite structure which can be nondestructively examined to determine the presence and extent of mechanical damage.
Another object of the invention is to provide a method for the nondestructive examination of composite structures.